I am so sorry about your brothers poor health and also for your difficulty finding your place in all this.
My thoughts are to be a loving sister, talk to him about life in general and do nice things that sisters do if you feel inclined, but detach from anything to do with his drugs and his glorification of using. You get to decide that part, your own sobriety is more important than anything he could possibly want from you.
I wouldn't help him get his drug or use it, there are some things I simply would not do and that's one of them, but that's me. You get to decide what is and what isn't okay for you.
Big hugs and prayers out for both of you.